landfills: Soil Covers Provide Alternatives for Closure

John Rohr

The U.S. Environmental Protection Agency may lay down the rules for landfill closure, but when it comes to final covers, there may be a natural alternative.

For example, landfill covers constructed of native soils and vegetation can achieve the required level of infiltration reduction and provide significant cost savings over the current prescribed cover design.

Two types of alternative cover designs are monolithic soil cover and capillary break cover systems.

The monolithic design uses one type of soil, which is typically silty, due to its high water holding capacity and compatibility with vegetation. This design works well in arid and semi-arid sites because of its high rate of evapotranspiration (between 80 percent and 100 percent).

The capillary break system is comparable to a monolithic cover, except for a coarse-grained material layer (i.e. gravel), which is inserted about 3 feet below the top soil layer. This lower layer disrupts soil suction, which often draws moisture downward. Although more expensive than the monolithic cover, this system can perform better in cold or wet climates.

Alternative final covers can provide several cost-saving benefits:

*On-site soils are used instead of importing large amounts of clay or buying synthetic materials.

*Landfill personnel and equipment can be used to construct the alternative cover, reducing operations and construction costs.

*Sites can cover as they fill, even one or two acres at a time.

*Native vegetation takes hold easier because of the more natural soil conditions.

*Long-term maintenance costs are lower.

In addition, geomembranes and compacted clay layers limit the activity possible at sites after closure and also limit how the land can be contoured or sloped.

Using an alternative cover offers more flexibility. For example, a site that is currently an active landfill can be turned into a golf course, driving range or other natural recreation/ greenspace areas after final closure.

Currently, the city of Glendale, Ariz., is testing an alternative cover design at a landfill it owns and operates, and is achieving positive results.

"We were concerned that the regulations suggested using clay in final cover design, which does not do well in our climate," says Michael Hoyt, field operations director for the city. "In a desert area with little water, clay has a tendency to crumble, which is the exact opposite of the intent of the regulations."

Glendale's landfill wasn't ready to begin the closure process, but wanted to identify its long-term costs. The city decided to test an alternative final cover with the help of Rust Environment & Infrastructure, Greenville, S.C., a consultant from the University of Wisconsin and the Arizona Department of Environmental Quality (DEQ).

Following DEQ approval, construction of two one-acre test covers began. One includes vegetation as the top layer while the other does not. Half of each test cover is soil only; the other half includes soil, plus a layer of 18 inches of gravel. An irrigation system supports the vegetation cover.

"All four configurations are functioning beyond what is required," Hoyt says.

A critical part of the project was to determine the performance criteria. Subtitle D does not specify how that should be determined, only that the alternative cover must perform equal to or better than the prescribed cover. Dr. Craig Benson from the University of Wisconsin and Rust established an equivalency test which was approved by the state agency.

"The analytical data shows that even when compared to the worst weather events over the past 15 years, all four alternative configurations are constantly pulling water up and out," Hoyt says.

Arizona DEQ is expected to issue their findings on the project later this year.